A Simultaneous Optical and X‐Ray Variability Study of the Orion Nebula Cluster. I. Incidence of Time‐correlated X‐Ray/Optical Variations

We present a database of $BVRI$ time-series photometry of the Orion NebulaCluster obtained with two ground-based telescopes at different longitudes toprovide simultaneous coverage with the 13-d Chandra observation of the cluster.The resulting database of simultaneous optical and X-ray light curves for some800 pre--main-sequence (PMS) stars represents, by a factor of hundreds, thelargest synoptic, multi--wavelength-regime, time-series study of young stars todate. This database will permit detailed analyses of the relationship betweenoptical and X-ray variability among a statistically significant ensemble of PMSstars, with the goal of elucidating the origins of PMS X-ray production. Inthis first paper, we present the optical observations, describe the combinedX-ray/optical database, and perform an analysis of time-correlated variabilityin the optical and X-ray light curves. We identify 40 stars (representing 5% ofour study sample) with possible time-correlated optical and X-ray variability.Examples of both positive and negative time-correlations are found, possiblyrepresenting X-ray flares and persistent coronal features associated with bothcool and hot surface spots (i.e. magnetically active regions and accretionshocks). We also find two possible examples of "white-light" flares coincidentwith X-ray flares; these may correspond to the impulsive heating phase insolar-analog flares. However, though interesting, these represent unusualcases. More generally, we find very little evidence to suggest a direct causallink between the sources of optical and X-ray variability in PMS stars. Theconclusion that accretion is a primary driver of X-ray production in PMS starsis not supported by our findings.Comment: 69 pages, 46 figures, accepted by Ap